Melt the butter in a saucepan, stir in the flour and cook for 1 minute.,Gradually blend in the milk and heat, stirring until the sauce thickens.,Stir in the salad cream, lemon juice and seasoning.,Place the fish in a shallow oven-proof dish (so that bottom of dish is covered).,Pour over the sauce, sprinkle evenly with the mushrooms.,______,Cook in a preheated oven (190C/375F) for 20 minutes, then place under a preheated grill for 2 minutes to brown the topping.,Garnish with tomato slices.